(+)-CC-1065 (1) and the duocarmycins represent the initial members of a class of exceptionally potent antitumor antibiotics that derive their biological effects through the reversible, stereoelectronically-controlled sequences alkylation of DNA (Boger et al. J. Org. Chem. 1990, 55, 4499; Boger et al. J. Am. Chem. Soc. 1990, 112, 8961; Boger et al. J. Am. Chem. Soc. 1991, 113, 6645; Boger et al. J. Am. Chem. Soc. 1993, 115, 9872; Boger et al. Bioorg. Med. Chem. Lett. 1992, 2, 759). Subsequent to their initial disclosure, extensive efforts have been devoted to establish their DNA alkylation selectivity and its structural origin. Efforts have also been devoted to establish the link between DNA alkylation and the ensuing biological properties, i.e., to define the fundamental principles underlying the relationships between structure, chemical reactivity, and biological properties (FIG. 1; 1-3).
CBI (1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one) has been identified as an alkylation subunit which corresponds to the alkylation subunit of CC-1065 and the duocarmycins. (Boger et al. J. Am. Chem. Soc. 1989, 111, 6461; Boger D. L.; Ishizaki et al. J. Org. Chem. 1990, 55, 5823). Agents which include the CBI-based analogs have proven especially useful as DNA alkylating agents. This was significant since preceding studies had attributed such unique characteristics to the naturally occurring alkylation subunits that they left the perception that even small structural perturbations, let alone deep-seated structural changes, would have detrimental effects on the properties. (Hurley et al. Science 1984, 226, 843; Reynolds et al. Biochemistry 1985, 24, 6228; Hurley et al. Biochemistry 1988, 27, 3886; Hurley et al. J. Am. Chem. Soc. 1990, 112, 4633; Warpehoski et al. Biochemistry 1992, 31, 2502.) Not only has this proven inaccurate, but the natural enantiomers of the CBI-based analogs of (+)-CC-1065 have proven chemically more stable (4.times.), biological more potent and considerably more synthetically accessible than the corresponding agents incorporating the natural CPI alkylation subunit of CC-1065. (Boger et al. J. Org. Chem. 1990, 55, 5823; Boger et al. J. Org. Chem. 1992, 57, 2873; Boger et al. J. Org. Chem. 1995, 60, 1271.) Moreover, selected agents within the series of CBI analogs not only exhibited potent cytotoxic activity but also potent and efficacious in vivo antitumor activity. (Boger et al. Bioorg. Med. Chem. Lett. 1991, 1, 115).
The natural enantiomers of the CBI-based analogs have been shown to alkylate DNA with an unaltered sequence selectivity at an enhanced rate and with a greater efficiency than the corresponding CPI analog. (Boger et al. Bioorg. Med. Chem. Lett. 1991, 1, 115; Boger et al. J. Am. Chem. Soc. 1991, 113, 2779; Boger et al. J. Am. Chem. Soc. 1992, 114, 5487.) This indicates that the simplified CBI alkylation subunit offers important advantages over the natural alkylation subunit of CC-1065. In recent studies, models of the DNA alkylation reactions of CC-1065 and the duocarmycins have been developed. (Boger et al. J. Org. Chem. 1990, 55, 4499; Boger et al. J. Am. Chem. Soc. 1990, 112, 8961; Boger et al. J. Am. Chem. Soc. 1991, 113, 6645; Boger et al. J. Am. Chem. Soc. 1993, 115, 9872; Boger et al. Bioorg. Med. Chem. Lett. 1992, 2, 759; Boger et al. J. Am. Chem. Soc. 1994, 116, 1635. ) These models accomodate the reversed and offset AT-rich adenine N.sub.3 DNA alkylation selectivity of the enantiomeric agents and their structural analogs. The diastereomeric adducts derived from the unnatural enantiomers have been found to suffer a significant destabilizing steric interaction between the CPI C7 center (CH.sub.3) or the CBI C8 center with the base adjacent to the alkylated adenine which is not present with the natural enantiomer adducts. Consistent with this observation, the distinguishing features between the natural and unnatural enantiomers diminish or disappear as the inherent steric bulk surrounding this center is reduced or removed. (Boger et al. J. Am. Chem. Soc. 1994, 116, 7996.) Because of the unnatural enantiomer sensitivity to destabilizing steric interactions surrounding the CPI C7 or CBI C8 center, the unnatural enantiomers of the CBI-based analogs are more effective than the corresponding CPI analog displaying an even more enhanced relative rate and efficiency of DNA alkylation.
There is a direct relationship between functional stability and cytotoxic potency. (Boger et al. J. Am. Chem. Soc. 1994, 116, 6461; Boger et al. J. Am. Chem. Soc. 1994, 116, 11335; Mohamadi et al. J. Med. Chem. 1994, 37, 232; Boger et al. J. Org. Chem. 1994, 59, 4943; Boger et al. J. Am. Chem. Soc. 1989, 111, 6461; Boger et al. J. Org. Chem. 1990, 55, 5823). In an ongoing series of studies conducted with agents containing deep-seated modifications in the alkylation subunit which to date include 4-9 (FIG. 2), the agents possessing the greatest solvolysis stability have been found to exhibit the most potent cytotoxic activity. Moreover, this direct relationship between functional stability and biological potency has been observed with both simple and advanced analogs of the natural products. A subsequent validation of this relationship was observed with a series of simple N.sup.2 substituted CBI derivatives. (Boger et al. J. Am. Chem. Soc. 1994, 116, 5523.) Predictable linear relationships between solvolysis stability (-log k), cytotoxic potency (log 1/IC.sub.50, L1210) and the electron-withdrawing properties of the N.sup.2 substituent (Hammett .sigma..sub.p constant) were observed (FIG 2; 4-9).
What is needed is an alternative alkylating agent having an altered reactivity as compared to CBI which may be incorporated into analogs of CC-10665 and the duocarjycins.